In recent years, development of an organic electroluminescent element to a display, illumination or the like has been progressing.
While an OLED is attractive in an excellent chromogenic property and an excellent energy-saving property, specific examples of a most significant challenge include poorness of durability. In particular, occurrence of a non-light-emitting portion called a dark-spot caused by moisture has posed a problem.
Consequently, as a method for allowing no invasion of moisture into an OLED, for example, Patent Literature 1 proposes a method in which an OLED is sealed inside a hollow airtight vessel to fill a space inside this airtight vessel with an inert gas atmosphere.
Moreover, for example, Patent Literatures 2 and 3 also propose a method in which a protective layer or the like composed of an inorganic substance having low moisture permeability typified by silicon nitride is provided on an OLED layer or a light-emitting part of an OLED.
However, according to a method of filling an inert gas inside the airtight vessel, a heat dissipation property is insufficient, and therefore such a method will adversely affect a life of an OLED, on the contrary. Therefore, the method has had a problem of incapability of sufficiently extending the life.
Moreover, formation of a film of the protective layer composed of the inorganic substance without a microscopic defect is significantly difficult by a current technology, and a problem of occurring the dark-spot under a high temperature and high humidity environment has still remained. In order to solve the above problem, it also may make attempt to form a protective layer having multilayer structure, for example by frequently repeating a method of forming a defect flattening layer on the protective layer composed of the inorganic substance, and then further stacking another protective layer composed of the inorganic substance thereon. However, application of such a method to a display and illumination for a general household has had a major challenge in view of labor and cost.
Patent Literatures 4 and 5 propose an art in which moisture-proof oil such as silicone oil or a curable resin is used as a filler in place of the inert gas.
However, the method in which the moisture-proof oil or the curable resin is filled therein has had a problem of generating gas bubbles inside the airtight vessel by outgassing caused by volatilization of a low molecular weight organic component, a solvent or the like contained in the moisture-proof oil or the curable resin to adversely affect aesthetic appearance (also referred to as visibility) of a display. Moreover, outgassing is permeated into organic molecules that form the OLED layer to cause a problem of an increase of a risk of occurring the dark-spot.
Furthermore, even if the moisture-proof oil or the curable resin is used in combination with the protective layer, an influence of moisture is unable to be completely eliminated, and the dark-spot is still occurred particularly under the high temperature and high humidity environment over a long period of time.
Therefore, for example, Patent Literatures 6 and 7 also propose a method in which hygroscopic metal oxide such as calcium oxide and magnesium oxide is added to a curable resin or the like to provide a filler with drying capability.
However, when the curable resin or the like is used as the filler, an OLED element is easily broken by cure shrinkage of the curable resin or the like, and a breakage risk during cure has posed a problem.
In the meantime, if the filler provided with the drying capability is transparent and colorless, such a filler can also be applied to a so-called “top emission type” OLED element in which light generated in the OLED element is emitted from a side of a sealing substrate (counter substrate), and not from side of a TFT substrate. In the top emission type OLED element, no emitted light is shielded by TFT. Therefore, in comparison with a so-called “bottom emission type” one, light-emitting efficiency is higher, and if “top emission type” one is applied to an image display apparatus, for example, “top emission type” one is advantageous in view of obtaining an image having higher luminance.
As such a filler that is transparent and colorless and has drying capability, for example, Patent Literatures 8 to 11 propose an organometallic compound such as aluminum oxide alkoxide and a mixture thereof with a resin.